A food packaging material requires being vapour and gas impermeable in order to achieve sufficient preservability of the product. In addition, the properties of the package may include that the product can be heated in a range or microwave oven. To meet these requirements, the board used for food package is usually coated at least from one side with one or several polymer layers so that the different layers provide different function in the board and the finished package. Due to its high melting temperature of about 250° C., polyethylene terephthalate (PET) has been the preferred polymer material for the coating of oven boards.
EP 2 222 462 B1 discloses a polymer-coated oven board which contains an oxygen barrier layer of polyamide (PA) and a surface layer of polyethylene terephthalate (PET) adhered to it, the layers having been applied onto the board by means of coextrusion, i.e. by extruding melts of the polymers and applied onto the board from a die. PA is aimed to strengthen the structure and to remedy the insufficient gas barrier lent by PET.
However, the prior art extrusion technique involves a number of limitations. According to said EP 2 222 462 B1 the weight of an extruded PET coating layer shall be 20 to 50 g/m2, preferably 30 to 40 g/m2. This technique thus requires a thickish PET layer for achieving sufficient bonding to the underlying PA layer in the coextrusion process. Thinning down the PET layer would result in failure of bonding with PA. Good adhesion between extruded PET and other polymer layers could be achieved by means of an intermediate tie-resin layer (e.g. modified ethylene acrylate resins like Bynell 22E804 from DuPont), but unfortunately such tie-resins lack sufficient heat resistance to be applicable for dual-ovenable food packaging.
Extruded PET coating layers do not crystallize, which is a further disadvantage in view of the barrier properties. An overmost thick PET layer also has a tendency of cracking at the bends and corners as the coated board is turned to trays and packages.
The problem thus is to find an improved technique for manufacturing an oven board, which fulfills the requirement of heat-resistance and a sufficient vapor and gas barrier, while avoiding the risk of cracking as the board is turned to trays or containers, and bringing saving of polymer material by reducing the required thickness of the PET coating layer.